Red-emitting organic light emitting devices (OLED&#39;s)

ABSTRACT

Dopant compounds of Formula I below for use in organic light emitting devices (OLED&#39;s) as device elements capable of emitting light of wavelengths associated with saturated red emissions.OLEDs utilize device elements comprising the above compounds and display devices are based on those OLED&#39;s.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/358,086, filed Jul. 21, 1999, now U.S. Pat. No. 6,329,085, which is acontinuation of U.S. application Ser. No. 08/774,087, filed Dec. 23,1996, now U.S. Pat. No. 6,048,630, which claims the benefit of U.S.Provisional Application No. 60/021,095, filed Jul. 2, 1996.

GOVERNMENT RIGHTS

This invention was made with Government support under contract No.F33615-94-1-1414 awarded by DARPA. The Government has certain rights inthe subject invention.

FIELD OF THE INVENTION

This invention relates, in general, to multicolor organic light emittingdevices (OLED's) and, more particularly, to combinations of dopants andhost compounds that enable the generation of a saturated red emissionsuitable for use in devices such as, for example, flat panel electronicdisplays.

BACKGROUND OF THE INVENTION

Electronic display devices are becoming an increasingly indispensabletool in modern society for the delivery of visual information. Thesedevices find widespread utility in television sets, computer terminals,and in a host of related applications. No other type of technologyoffers comparable speed, versatility and potential for interactivity.Current electronic display technologies include, for example, cathoderay tubes (CRT's), plasma displays, light emitting diodes (LED's), thinfilm electroluminescent displays, and the like.

The most widely-used non-emissive technology for display devices makesuse of the electro-optic properties of a class of organic moleculesknown as liquid crystals in fabricating liquid crystal displays (LCD's).LCD's operate fairly reliably, but are limited by relatively lowcontrast, low speed, and fade-out when viewed from oblique angles, aswell as the requirement for high power backlighting. Active matrixdisplays, in a partial solution to these shortcomings, employ an arrayof transistors, each capable of activating a single liquid crystalpixel, thus improving contrast.

There is no doubt that flat panel display technology is of significantscientific and commercial interest. Consequently, it is the subject ofextensive ongoing research. See Depp, S. W. and Howard, W. E., “FlatPanel Displays,” Scientific American, March 1993, pps. 90-97. Accordingto Depp and Howard, by 1995, flat panel displays alone were expected togenerate a market of between $4 and $5 billion. Key to the success ofany potential display technology in this market is the ability to bothprovide a high resolution, full-color display at good light level and,at the same time, to be competitively priced.

Organic thin film materials represent a technical development that hasdemonstrated considerable progress in the fabrication of red, green andblue light emitting devices. These organic light emitting devices havebeen shown to have sufficient brightness, range of color and operatinglifetimes for use as a practical alternative technology to LCD-basedfull color flat-panel displays (S. R. Forrest, et al., Laser FocusWorld, February 1995). Furthermore, since many of the organic thin filmsused in such devices are transparent in the visible spectral region,they potentially allow for the realization of a completely new type ofdisplay pixel in which the red (R), green (G), and blue (B) emissionlayers are placed in a vertically stacked geometry to provide a simplefabrication process, minimum R-G-B pixel size, and maximum fill factor.

Disclosed in U.S. Pat. No. 5,294,869 to C. W. Tang and J. E. Littman isa concept for using separate, side-by-side red, green, and blue OLED'sto make a full color display. However, it is believed by the inventorsof the instant disclosure that such concepts have never beensuccessfully realized in a practical device.

Such schemes suffer from a complex layer structure, and lack of knownmethods for damage-free, post-deposition patterning of organic layers atthe resolution required for color displays. Others have alternativelysuggested using an array of white OLED's (J. Kido, et al., Science 267,1332 (1995)) backed by side-by-side R, G and B color filters depositedand patterned prior to OLED growth. However, such a design sacrifices atleast 66% of the light from each white OLED, with the remainder beingabsorbed in the filter, also generating heat. Such a design suffers,therefore, from low efficiency and conditions of accelerateddegradation. Alternative schemes based on micro cavity filtering of abroad-spectrum OLED (A. Dodabalapur, et al., Appl. Phys. Lett. 64, 2486(1994)) suffer from complex and expensive substrate patterningrequirements and extremely limiting directionality of the resultingcolor pixels.

An example of a multicolor electroluminescent image display deviceemploying organic compounds for light emitting pixels is disclosed inTang et al., U.S. Pat. No. 5,294,870. This patent discloses a pluralityof light emitting pixels which contain an organic medium for emittingblue light in subpixel regions. Fluorescent media are laterally spacedfrom the blue-emitting subpixel region. The fluorescent media absorblight emitted by the organic medium and, in turn, emit red and greenlight in different subpixel regions. The use of materials doped withfluorescent dyes to emit green or red on absorption of blue light fromthe blue subpixel region is less efficient than direct formation viagreen or red LED's. The reason is that the efficiency will be theproduct of (quantum efficiency for EL) and (quantum efficiency forfluorescence) and (1-transmittance). Thus, a drawback of this displayand all displays of this type is that different laterally spacedsubpixel regions are required for each color emitted.

Color-tunable OLED's potentially allow for full-color operation withoutthe complex structures common to other types of devices. Publishedexamples of tunable OLED's utilize a blend of either two polymers (M.Granstrom and O. Inganas, Appl. Phys. Lett. 68, 147 (1996)) or a polymerdoped with semiconductor nanocrystallites (B. O. Dabbousi, et al., Appl.Phys. Lett. 66, 1316 (1995); V. L. Colvin, et al., Nature 370, 354(1994)). Each component of the blend emits radiation having a differentspectral energy distribution. The color is tuned by varying the appliedvoltage. A higher voltage results in more emission from the higherbandgap polymer, which emits radiation toward the blue region of thespectrum, while also resulting in higher overall brightness due toincreased current injection into the device. Although tuning from orangeto white has been demonstrated, incomplete quenching of the low-energyspectral emission appears to prohibit tuning completely into the blue.In addition, emission intensity can only be controlled by using pulsedcurrent and reduced duty cycles. In a color display, therefore,prohibitively high drive voltages and very low duty cycles may benecessary for blue pixels. This necessitates a complex driver circuit,renders passive matrix operation extremely difficult, if not impossible,and is likely to accelerate degradation of the display.

A transparent organic light emitting device (TOLED) which represents afirst step toward realizing high resolution, independently addressablestacked R-G-B pixels has been reported recently in the publishedinternational patent application No. WO 96/19792. This TOLED had greaterthan 71% transparency when turned off, and emitted light from both topand bottom device surfaces with high efficiency (approaching 1% quantumefficiency) when the device was turned on. The TOLED used transparentindium tin oxide (ITO) as the hole-injecting electrode layer, and aMg:Ag-ITO layer for electron injection. A device was disclosed in whichthe Mg:Ag-ITO electrode was used as a hole-injecting contact for asecond, different color-emitting OLED stacked on top of the TOLED. Eachdevice in the stack was independently addressable and emitted its owncharacteristic color through the transparent organic layers, thetransparent contacts and the glass substrate allowing the entire devicearea to emit any combination of color that could be produced by varyingthe relative output of the two color-emitting layers.

Thus, for the specific device disclosed in WO 96/19792, which included ared-emitting layer and a blue-emitting layer, the color output producedby the pixel could be varied in color from deep red through blue.

It is herein believed that WO 96/19792 provided the first demonstrationof an integrated OLED where both intensity and color could beindependently varied by using external current sources. As such, WO96/19792 represents the first proof-of-principle for achievingintegrated, full color pixels which provide the highest possible imageresolution (due to the compact pixel size) and low cost fabrication (dueto the elimination of the need for side-by-side growth of the differentcolor-producing pixels).

Presently, a frequently used high-efficiency organic emissive structureis one referred to as the double heterostructure LED which is known tothose of skill in the appropriate art. This structure is very similar toconventional, inorganic LED's using materials as GaAs or InP. In thistype of device, a support layer of glass is coated by a thin layer ofindium/tin oxide (ITO) to form the substrate for the structure. Next, athin (100-500 Å) organic, predominantly hole-transporting, layer (HTL)is deposited on the ITO layer. Deposited on the surface of the HTL layeris a thin (typically, 50-100 Å) emissive layer (EL). If these layers aretoo thin, there may be breaks in the continuity of the film; as thethickness of the film increases, the internal resistance increases,requiring higher power consumption for operation. The range of 100-1000Å represents the best typical compromise between these extremes. Theemissive layer (EL) provides the recombination site for electrons,injected from a 100-500 Å thick electron transporting layer (ETL) thatis deposited upon the EL, and holes from the HTL layer. The ETL materialis characterized by considerably higher mobility for electrons than forcharge deficient centers (holes). Examples of prior art ETL, EL and HTLmaterials are disclosed in U.S. Pat. No. 5,294,870 entitled “OrganicElectroluminescent MultiColor Image Display Device,” issued on Mar. 15,1994 to Tang et al., the disclosure of which is hereby incorporated byreference.

Often, the EL layer is doped with a highly fluorescent dye to tune thefrequency of the light emitted (color), and increase theelectroluminescence efficiency of the LED. The double heterostructuredevice described above is completed by depositing metal contacts ontothe ITO Layer, and a top electrode onto the electron transporting layer.The contacts are typically fabricated from indium or Ti:Pt:Au. Theelectrode is often a dual-layer structure consisting of an alloy such asMg:Ag directly contacting the organic ETL layer, and a thick, opaquesecond layer of a high work function metal such as gold (Au) or silver(Ag) on the Mg:Ag or the transparent Mg:Ag/ITO electrode. When properbias voltage is applied between the top electrode and the metalcontacts, light emission occurs through the glass substrate for deviceswith an opaque top electrode, and through both surfaces for transparentOLED's. An LED device of this type typically has luminescent externalquantum efficiencies of from 0.05 percent to 4 percent depending on thecolor of emission and the device structure.

Another known organic emissive structure is referred to as a singleheterostructure. The difference in this structure relative to that ofthe double heterostructure is that the electroluminescent layer alsoserves as an ETL layer, eliminating the need for the ETL layer. However,this type of device, for efficient operation, must incorporate an ELlayer having good electron transport capability, otherwise a separateETL layer must be included, rendering the structure effectively the sameas a double heterostructure.

Presently, the highest efficiencies have been observed in green LED's.Furthermore, drive voltages of 3 to 10 volts have been achieved. Theseearly and very promising demonstrations have used amorphous or highlypolycrystalline organic layers. These structures undoubtedly limit thecharge carrier mobility across the film which, in turn, limits currentand increases drive voltage. Migration and growth of crystallitesarising from the polycrystalline state is a noted failure mode of suchdevices. Electrode contact degradation is also a common mechanism offailure.

A known alternative device structure for an LED is referred to as asingle layer (or polymer) LED. This type of device includes a glasssupport layer coated by a thin ITO layer, forming the base substrate. Athin organic layer of spin-coated polymer, for example, is then formedover the ITO layer, and provides all of the functions of the HTL, ETL,and EL layers of the previously described devices. A metal electrodelayer is then formed over the organic polymer layer. The metal istypically Mg, Ca, or other conventionally used metals.

Devices whose structure is based upon the use of layers of organicoptoelectronic materials generally rely on a common mechanism leading tooptical emission. Typically, this mechanism is based upon the radiativerecombination of a trapped charge. Specifically, devices constructedalong the lines discussed above comprise at least two extremely thinorganic layers separating the anode and cathode of the device. Thematerial of one of these layers is specifically chosen based on thematerial's ability to transport holes (the HTL layer); the other,according to its ability to transport electrons (the ETL or EL layer).This last layer typically comprises the electroluminescent layer. Withsuch a construction, the device can be viewed as a diode with a forwardbias when the potential applied to the anode is higher than thepotential applied to the cathode. Under these bias conditions, the anodeinjects holes (positive charge carriers) into the electroluminescentlayer, while the cathode injects electrons into the EL layer. Theportion of the luminescent medium adjacent to the anode thus forms ahole injecting and transporting zone while the portion of theluminescent medium adjacent to the cathode forms an electron injectingand transporting zone. The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, a Frenkel exciton is formed.Recombination of this short-lived state may be visualized as an electrondropping from its conduction potential to a valence band, withrelaxation occurring preferentially, under certain conditions, via aphotoemissive mechanism. Under this view of the mechanism of operationof typical thin-layer organic devices, the electroluminescent layercomprises a luminescence zone receiving mobile charge carriers(electrons and holes) from each electrode.

A specific example of a red-emitting device is disclosed in U.S. Pat.No. 5,409,783 to Chang et al. The disclosed device utilizes an EL layerof tris(8-quinolinol) aluminum doped with magnesium phthalocyanine toachieve a red emission suitable for use in printing on colorphotographic paper. However, according to FIG. 5 of the reference, theemission spectrum of such a device, while displaying an emission maximumat approximately 690 nm, also displays a significant emission maximum at530 nm (green). Although this apparently does not pose a problem forapplications such as color printing, mentioned in Example 1 of thereference, such an emission would be unacceptable for use in a pixeldevice for a visual display in that the red light emission would behighly unsaturated.

SUMMARY OF THE INVENTION

The present invention is generally directed to a multicolor organiclight emitting device, and structures containing the same, employing anemissive layer comprising emitting compounds selected for the emissionof desirable primary colors. Included among these compounds are thosedisclosed in Applicants' co-pending U.S. patent application Ser. No.08/774,120, filed Dec. 23, 1996, now U.S. Pat. No. 5,811,833, and Ser.No. 08/693,359, filed Aug. 6, 1996, now U.S. Pat. No. 6,358,631, thedisclosures of which are hereby incorporated by reference. Inparticular, the present invention is directed to the use of a selectgroup of dopants which, when added in effective amounts to suitablereceiving compounds, including emitting compounds and/or host compounds,enhance the emission of red light.

The dopants for use in the present invention include compounds ofFormula I (see also FIG. 1):

wherein X is C or N;

R₈, R₉, and R₁₀ are each independently selected from the groupconsisting of hydrogen; alkyl; substituted alkyl; aryl; substitutedaryl, with substituents selected from the group consisting of loweralkyls, halogens and recognized donor and acceptor groups;

R₉ and R₁₀ may be combined together to form a fused ring;

M₁ is a divalent, trivalent or tetravalent metal;

a, b and c are each 0 or 1;

wherein, when X is C, then a is 1; when X is N, then a is 0;

when c is 1, then b is 0; and when b is 1, c is 0.

Also contemplated are acetylenyl bis-molecular arrays of the compoundsof Formula I, represented, in general, by the formula I—C≡C—I, asdisclosed in Lin, V. S. -Y.; Therien, M. J., Science 264, 1105-1111(1994).

The dopants for use in the present invention are incorporated into ahost compound so that the LED device emits light that is perceived bythe human eye to be close to a saturated red color. Through the practiceof the present invention, it is possible to construct OLED'scharacterized by an emission that is closer to an absolute (orsaturated) value, as that would be defined by the CIE scale, forexample, than heretofore possible. Furthermore, LED's utilizing thematerials of the present invention are also capable of a displaybrightness that can be in excess of 100 cd/m².

The receiving compounds as defined herein are any compounds which can bedoped with the dopants defined above to emit light with the desiredspectral characteristics. Such compounds include, but are not limitedto, both emitting compounds and host compounds as described in U.S.patent application Ser. No. 08/693,359, filed Aug. 6, 1996, now U.S.Pat. No. 6,358,631, incorporated herein by reference.

For example, emitting compounds can include compounds of Formula II:

wherein M is a trivalent metal ion;

Q is at least one fused ring, at least one of said fused ringscontaining at least one nitrogen atom;

and L is a ligand selected from the group consisting of:

picolylmethylketone; substituted and unsubstituted salicylaldehyde;

a group of the formula R¹(O)CO,

wherein R¹ is selected from the group consisting of hydrogen, an alkylgroup, an aryl group, and a heterocyclic group, each of which may besubstituted with at least one substituent selected from the groupconsisting of aryl, halogen, cyano and alkoxy;

halogen;

a group of the formula R¹O, wherein R¹ is as defined above;

bistriphenyl siloxides; and quinolates and derivatives thereof;

p is 1 or 2; and t is 1 or 2, where p does not equal t.

Emitting compounds can also be of Formulas III and IV:

wherein Q is defined as above for both III and IV, and where M isdivalent for III and trivalent for IV. Exemplary compounds coming withinFormulas II and IV are illustrated in FIGS. 2A-2I, and 3A-3M,respectively. Other compounds suitable as emitting compounds forreceiving the dopants are shown in FIGS. 4A-10J. Specific examples ofcompounds of Formula III, as well as information on the preparation ofsuch compounds, as readily comprehended by one of skill in the art, canbe found in the following publications: Hamata, Y., et al., Jpn. J.Appl. Phys. 32 (1993) Pt. 2, No. 4A, L511-L513; Hamata, Y., et al.,Chem. Lett. 1993, 905-906; Hamata, Y., et al., Jpn. J. Appl. Phys. 32(1993) Pt. 2, No. 4A, L514-L515.

Host compounds which facilitate the carrying of charge or excitation tothe emitting compounds to initiate light emission and which can alsoreceive dopants in accordance with the present invention includecompounds of Formulas V-VIII as shown below, and in FIGS. 12A-12D:

wherein M for compounds represented by formulas V and VI is a +3 metalion, preferably aluminum, gallium or indium, while M for compoundsrepresented by formulas VII and VIII is a +2 metal ion, preferably zinc;and R₁ through R₅ are each independently selected from the groupconsisting of hydrogen, alkyl and aryl.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings pertaining to the present invention, in whichlike reference characters indicate like parts, are illustrative ofembodiments of the invention, and are not intended to limit the scope ofthe invention as encompassed by the claims forming part of theapplication.

FIG. 1 represents a structural formula of dopants for use in lightemitting devices in accordance with the present invention;

FIGS. 2A-10J are structural formulas of emitting compounds which can beused to receive dopants in accordance with the present invention;

FIGS. 11A-11D are structural formulas of host compounds which can beused to receive dopants in accordance with the present invention;

FIG. 12 is a cross-sectional view of an OLED device that can be employedin accordance with the present invention.

FIG. 13A is a cross-sectional view of a typical organic doubleheterostructure LED for use with the present invention; FIG. 13B is across-sectional view of a typical organic single heterostructure LED foruse with the present invention; FIG. 13C is a cross-sectional view of aknown single-layer polymer LED structure for use with the presentinvention;

FIG. 14 is a cross-sectional view of an integrated three-color pixelutilizing crystalline organic light emitting devices (LED's) for usewith the present invention;

FIG. 15 is a plot of the emission spectrum of an embodiment of a redemitting layer of the present invention;

FIG. 16 is a plot showing the relationship between optical power andcurrent obtained from an embodiment of the present invention;

FIG. 17 is a cross-sectional view of an alternative embodiment of anOLED for use with the present invention for emitting both saturated redlight and blue light; and

FIG. 18 is plot of the spectra at various voltages for a stacked,multiple-element OLED according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to the addition of dopantsto suitable receiving compounds as defined herein to generate anemission of light in the red region of the visible spectrum (i.e. about650 nm) when such doped materials are used in functional layers of lightemitting devices (LED's). The present invention is particularly adaptedto organic light emitting devices (OLED's) which emit light that appearsto the typical observer as being close to saturated red.

The emission of red light in light emitting devices has beenaccomplished previously with emitting compounds that emit light in theregion of 640 nm to 740 nm. Clarity of the emission, however, has beenproblematical because the light is emitted at wavelengths which deviatesignificantly from those associated with saturated red (i.e., <650 nm).For example, emissions at about 675 nm are in the deep red region, whileemissions somewhat below 650 nm generally result in a faded redemission. It is therefore desirable to formulate an OLED which iscapable of emitting saturated red light with wavelengths in a regionclosely defined around 650 nm. For comparison purposes, the CIE colorcoordinates for a red video signal, according to the InternationalTelecommunications Union (ITU) would be x=0.6430; y=0.330; and z=0.030.

In accordance with the present invention, a dopant, capable of shiftingthe emission wavelength of a receiving compound, is added to a receivingcompound in an amount effective to shift the wavelength of emission sothat the LED device emits light that is perceived by the human eye to beclose to a saturated red color. Although it would be recognized that thecharacterization of color perceptions is an extremely subjectiveexercise, a quantitative chromicity scale has been developed by theCommission Internationale de l'Eclairage (International Commission ofIllumination), otherwise known as the CIE standard. According to thisstandard, a saturated red color would be represented by a single point,with specific quantitative coordinates according to the defined axes ofthe chromaticity scale. It will be appreciated by one of skill in theart that such a single point on the CIE scale would represent a standardor a goal that, in practical terms, is difficult, but fortunately,unnecessary, to attain. However, through the practice of the presentinvention, it is possible to construct OLED's characterized by anemission that is closer to an absolute (or saturated) value, as thatwould be defined by the CIE scale, than heretofore possible. Examples ofsuitable receiving compounds including emitting compounds and/or hostcompounds are of the type shown and described in Applicants' co-pendingU.S. patent application Ser. No. 08/693,359, filed Aug. 6, 1996, nowU.S. Pat. No. 6,358,631.

As the term receiving compound is used herein, it will be appreciatedthat such compounds can be found in either an electrontransporting/emissive layer (ETL/EL) of a single heterostructure LEDdevice, or in the emissive layer of a double heterostructure device. Aswill be recognized by one of skill in the art, the use of dopant speciessuch as disclosed herein makes it possible to both extend the range ofcolors emitted by the EL or ETL/EL of an OLED, but also to extend therange of possible candidate species for receiving compounds.Accordingly, for effective receiving/dopant systems, although thereceiving compound can have a strong emission in a region of thespectrum where the dopant species strongly absorbs light, the receivingspecies cannot have an emission band in a region where the dopant alsoemits. In structures where the receiving compound also functions as acharge carrier (combined ETL/EL), then additional criteria such as redoxpotential for the species also becomes a consideration. In general,however, the spectral characteristics of the receiving and dopantspecies are the most important criteria.

Dopants for use in the present invention are represented by compounds ofFormula I:

wherein X is C or N;

R₈, R₉, and R₁₀ are each independently selected from the groupconsisting of hydrogen; alkyl; substituted alky; aryl; substituted aryl,with substituents selected from the group consisting of lower alkyls,halogens and recognized donor and acceptor groups;

R₉ and R₁₀ may be combined together to form a fused ring;

M₁ is a divalent, trivalent or tetravalent metal;

a, b and c are each 0 or 1;

wherein, when X is C, then a is 1; when X is N, then a is 0;

when c is 1, then b is 0; and when b is 1, c is 0.

The preferred metals for M¹ (when c=1) are metals from either thetransition metal group or main groups of the Periodic Table.

Also contemplated are acetylenyl bis-molecular arrays of the compoundsof Formula I, represented, in general, by the formula I—C≡C—I, asdisclosed in Lin, V. S. -Y.; Therien, M. J., Science 264, 1105-1111(1994).

Preferred compounds of Formula I include those where X is carbon (i.e. ais 1), R is phenyl, C is 0, and b is 1. An example of a preferredcompound of Formula 1 is (5,10,15,20 tetraphenyl-21H,23H-porphine).Compounds of Formula I can be made according to procedures detailed inthe technical literature, as would be recognized by one of skill in therelevant art.

The amount of the dopants of the compounds of Formulas I and II for usein the present invention is an amount sufficient to shift the emissionwavelength of the red emitting material as close as possible tosaturated red as that would be defined according to the CIE scale.Typically, the effective amount is from about 0.01 to 10.0 mol %, basedon the emitting layer. The preferred amount is from about 0.1 to 1.0 mol%. However, it will be recognized here that the sole criterion fordetermining an appropriate doping level is the level effective toachieve an emission with the appropriate spectral characteristics. Byway of example, and without limitation, if the amount of dopant speciesis at too low a level, then emission from the device will also comprisea component of light from the host compound itself, which will be atshorter wavelengths than the desired emission form the dopant species.In contrast, if the level of dopant is too high, emission efficienciescould be adversely affected by self-quenching, a net non-emissivemechanism. Alternatively, too high levels of the dopant species couldalso adversely affect the hole or electron transporting properties ofthe host material.

The receiving compounds for receiving the dopant species of the presentinvention include compounds capable for forming a continuous film andwhich have an energy gap greater than that of the dopant. Typicalexamples of such compounds include emitting compounds of the typecovered by Formulas II-IV:

wherein M is a trivalent metal ion for II and IV, and a divalent metalion for III;

Q is at least one fused ring, at least one of said fused ringscontaining at least one nitrogen atom;

and L is a ligand selected from the group consisting of:

picolylmethylketone; substituted and unsubstituted salicylaldehyde;

a group of the formula R¹(O)CO,

wherein R¹ is selected from the group consisting of hydrogen, an alkylgroup, an aryl group, and a heterocyclic group, each of which may besubstituted with at least one substituent selected from the groupconsisting of aryl, halogen, cyano and alkoxy;

halogen;

a group of the formula R¹O, wherein R¹ is as defined above;

bistriphenyl siloxides; and quinolates and derivatives thereof;

p is 1 or 2; and t is 1 or 2, where p does not equal t.

The preferred trivalent metals are aluminum, gallium, and indium; thepreferred divalent metal is zinc.

The receiver compound also includes host compounds of the type coveredby Formulas V-VIII:

Preferred emitting compounds are those covered by Formulas IX and X:

wherein R is selected from the group consisting of hydrogen, an alkylgroup, an aryl group, and a heterocyclic group,

each of which may be substituted with at least one substituent selectedfrom the group consisting of: aryl, halogen, cyano and alkoxy;

M is a trivalent metal ion;

L is a ligand selected from the group consisting of:

picolylmethylketone, substituted and unsubstituted salicylaldehyde, agroup of the formula R¹(O)CO,

wherein R¹ is selected from the group consisting of:

hydrogen, an alkyl group, and a heterocyclic group, each of which may besubstituted with at least one substituent selected from the groupconsisting of:

aryl, halogen, cyano and alkoxy;

halogen;

a group of the formula R¹O, wherein R¹ is as defined above;

BIS(triphenyl) siloxides;

and quinolates and derivatives thereof;

A is an aryl group or a nitrogen-containing heterocyclic group fused tothe existing fused ring structure;

n₁ and n₂ are independently 0, 1, or 2; and m₁ and m₂ are independently1, 2, 3, and 4.

Also preferred as emitters are compounds of Formulas XI and XII:

wherein R, M, A, n1, n2, m1, and m2 are as defined above.

FIG. 12 shows a prior art OLED structure that can be utilized with thedopants and the receiver compounds of the present invention to emitsaturated red light. A device such as that shown in FIG. 12, when usedin the practice of the present invention, would have a 1-mm glass layer40 coated with 2000 Å of indium-doped tin oxide (ITO) 50 as a devicesubstrate. Deposited on the ITO layer is a 300 Å-thick hole transportinglayer (HTL) 60 comprised of a receiver compound such as TPD(N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine).There is then deposited a 500 Å-thick layer of a receiver compound dopedwith a compound of the present invention 70, such as5,10,15,20-tetraphenyl-21H,23H-porphine (TPP) at a level of 3 mol %.This structure is then covered by a two-layer electrode, with a Mg:Agalloy as the first layer 80, covered by Ag 90.

Other device structures which can be used in accordance with the presentinvention include, but are not limited to, those shown in FIGS. 13A-13Cand FIG. 14, each of which are described in detail in Applicants'co-pending application Ser. No. 08/693,359, filed Aug. 6, 1996, now U.S.Pat. No. 6,358,631, incorporated herein by reference. In addition, thesubject invention as disclosed herein may be used in conjunction withthe subject matter of co-pending applications, “High Reliability, HighEfficiency, Integratable Organic Light Emitting Devices and Methods ofProducing Same,” Ser. No. 08/774,119 (filed Dec. 23, 1996), now U.S.Pat. No. 6,046,543; “Novel Materials for Multicolor LED's,” Ser. No.08/850,264 (filed May 2, 1997), now U.S. Pat. No. 6,045,930; “ElectronTransporting and Light Emitting Layers Based on Organic Free Radicals,”Ser. No. 08/774,120 (filed Dec. 23, 1996), now U.S. Pat. No. 5,811,833;“Multicolor Display Devices,” Ser. No. 08/772,333 (filed Dec. 23, 1996),now U.S. Pat. No. 6,013,982; and “High Efficiency Organic Light EmittingDevice Structures,” Ser. No. 08/772,332 (filed Dec. 23, 1996), now U.S.Pat. No. 5,834,893; each of said co-pending applications or patentsbeing herein incorporated by reference in their entirety. The subjectinvention may also be used in conjunction with the subject matter ofco-pending U.S. Ser. No. 08/354,674, now U.S. Pat. No. 5,707,745; Ser.No. 08/613,207, now U.S. Pat. No. 5,703,436; Ser. No. 08/632,316, nowU.S. Pat. No. 5,721,160; Ser. No. 08/632,322, now U.S. Pat. No.5,757,026; Ser. No. 08/693,359, now U.S. Pat. No. 6,358,631; No.60/010,013, which provides priority for U.S. Pat. No. 5,986,268; and No.60/024,001, which provides priority for U.S. Pat. No. 5,844,343; each ofwhich are also herein incorporated by reference in their entirety.

EXAMPLES Example 1 Construction of a Red-Emitting OLED

Referring to FIG. 12, there is shown an OLED device particularly adaptedfor emitting saturated red light. The red OLED is grown on a glasssubstrate pre-coated with a transparent indium tin oxide (ITO) thin filmwith a nominal sheet resistivity of about 20 Ω/square. Prior todeposition of the organic films, the substrates are pre-cleaned byultrasonication in a mixture of isopropyl alcohol and water (1:1) anddegassed in toluene vapor. The red OLED device is formed by sequentialevaporation of a 600 Å-thick layer of a receiver material such as Alq₃(tris(8-hydroxyquinolato)aluminum), and a 600 Å-thick layer consistingof approximately 3 mol % of 5,10,15,20 tetraphenyl-21H,23H-porphine(TPP). TPP represents a compound falling within Formula I, wherein c=0;b=1; R₈ is phenyl; X═C; and R₉═R₁₀═H. TPD represents a compound fallingwithin Formula II, wherein R₁₁ is hydrogen, and R₁₂ is methyl.

Suitable receiving materials for use in devices such as those describedimmediately above would be exemplified by a compound of the formulagiven below:

wherein M is aluminum and R is hydrogen, prepared by usingco-evaporation techniques known to those of skill in the chemicallydeposited thin film arts. Light with a wavelength of about 650 nm isemitted from these devices when an external voltage, V, is appliedthrough a power source such as a battery. The applied external voltageis typically in the range of from about 3 to about 25 V, depending onthe brightness required.

The output spectrum of a red-emitting OLED with a structure according tothat of FIG. 12 is shown in FIG. 15. As that spectrum illustrates,output of light is essentially zero at wavelengths less than about 640nm, with a strong emission band at about 650 nm.

FIG. 16 shows the optical power measured through the glass substrate asa function of drive current. As this Figure illustrates, there is anessentially linear relationship between the optical power output of thedevice of the Example as a function of current, demonstrating theefficiency of the red-emitting device utilizing the present invention.At 100 μA drive current, the measured quantum efficiency from a 1-mmcircular device is approximately 0.14%, after allowing for lost lightwaveguided to the edge of the glass substrate.

Example 2 Construction of a Stacked Two-Element (Blue and Red) OLED

FIG. 17 shows a multi-layer device including a red OLED with a TPPcontaining emitter layer stacked upon a blue OLED.

The device consists of a red OLED grown on top of a blue TOLED. TheSOLED is grown on a glass substrate pre-coated with a transparent indiumtin oxide (ITO) thin film with a nominal sheet resistivity of 20Ω/square. Prior to deposition of the organic films, the substrates arepre-cleaned as described above. A blue TOLED is grown by sequentialevaporation, in a vacuum of <10⁻⁶ Torr, of three, pre-purified organicmaterials: a 600 Å-thick layer of hole conductingN,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD),followed by an 800 Å-thick layer of blue fluorescentbis-(8-hydroxy)quinaldine aluminum phenoxide (Alq₂OPh), and a 360Å-thick layer of electron-conducting tris (8-hydroxyquinolate aluminum)(Alq₃). The top, electron-injecting electrode consists of a 130 Å-thickMg:Ag alloy (in an approximate atomic ratio of 50:1), deposited bythermal evaporation. This is immediately capped by a second, 500 Å-thickITO layer sputter-deposited onto the Mg:Ag surface. The ITO layerprovides an electrically continuous, transparent conducting surface ontop of the thin metal film, protects the Mg-Ag electrode from oxidation,and serves as a hole injecting contact for a second OLED. Thetransparency of this Mg:Ag-ITO electrode in the visible spectral regionfor this particular device is approximately 50%. The second device isformed by sequential evaporation of a 600 Å-thick layer of TPD and a 600Å-thick layer consisting of 1 mol % (3 mass %) of5,10,15,20-tetraphenyl-21H,-23H-porphine (TPP) doped intotris(8-hydroxyquinilato) aluminum (Alq₃) using co-evaporation. Through asecond shadow mask, a 1500 Å-thick layer of Mg:Ag is grown, and cappedwith a 500 Å-thick layer of Ag to inhibit top electrode oxidation.

All layers of the resulting stacked device are transparent with theexception of the top electrode, allowing for the extension of the devicegeometry to three or more colors. Alq₃ and TPD are highly transparent atwavelengths longer than 420 nm and, although TPP has several π-{dot over(π)} absorption peaks (Q-bands) in the green spectral region, the-smallconcentration of TPP in Alq₃ (3 mass %) renders any absorption in theTPP/Alq₃ layer negligible. The absorption spectrum of the Mg:Ag-ITOelectrode shows no strong absorption features in the visible spectralregion, and a demonstrated transparency of up to 81%. There is also noevidence of photoluminescence pumping of the red element by the blueelement, again due to the negligible absorption of the organic layers inthe visible spectral region.

The device is operated by connecting the central Mg:Ag-ITO electrode toa common ground. The top Mg:Ag electrode and bottom ITO electrode arethen biased negative and positive (V_(R) and V_(B)), respectively.Output spectra from the device at various drive voltages are shown inFIG. 18. The spectra are measured by focusing light from the substratesurface of the device onto the entrance slit of a 0.275 m focal lengthspectrograph with an EG&G Optical Multichannel Analyzer at the outputport. The device emission spectrum can be varied to be any linearsuperposition of the red (peak wavelength at 655 nm) and blue (peakwavelength at 470 nm) emission by independently varying the ratio of thedrive voltage (V_(R)/N_(R)) of each element. Typical operatingconditions for devices of about 1.5 mm in diameter are 0.1-1 mA, andfrom about 15 to 25 V drive voltage, to obtain an optical power outputof from about 5-10 μW. FIG. 18 clearly shows that the light emissionfrom each element in the device may be varied independently of theother, allowing for continuous color tuning between the extremes ofcolor offered by the individual elements.

To estimate the quantum efficiency of the elements, a large-area silicondetector was used to measure the optical power through the glasssubstrate as a function of drive current. The quantum efficiencies oflight emission were calculated to be 0.07% for the red and 0.49% for theblue elements, assuming that 50% of the light from the red pixel elementis absorbed in the central electrode. The corresponding brightness is 42cd/m² for the red element, and greater than 200 cd/m² for the blueelements, sufficient for video display device applications.

Using the basic teachings disclosed above, other OLED-based devices canbe fabricated using the present invention in structures such as shown inFIG. 15.

What is claimed is:
 1. A light emitting device (LED) capable of use inan LED structure, the LED comprising: an emission layer, the emissionlayer comprising at least one receiving host compound and an emittingdopant compound having a structure represented by Formula I:

wherein R₈, R₉, and R₁₀ are each independently selected from the groupconsisting of hydrogen, alkyl, substituted alkyl, aryl and substitutedaryl; R₉ and R₁₀ may be combined together to form a fused ring; and M¹is a divalent, trivalent or tetravalent metal.
 2. A member of the groupconsisting of display, vehicle, television, computer, printer, screen,sign, and telecommunications device including a plurality of lightemitting devices in accordance with claim 1.